Refining of liquid hydrocarbons and fractionation provides a series of streams of hydrocarbon products. A stream such as hydrocarbon feed, unhydrotreated pyrolysis gasoline from steam cracker, FCC naphtha or unhydrotreated coker naphtha is further refined to provide a blend of C6-C8 aromatics, commonly referred to as BTX, comprising primarily benzene, toluene, ethylbenzene, styrene and a mixture of xylenes. For example, a process for production of BTX from FCC naphtha is described by Timken et al. in U.S. Pat. No. 5,685,972 issued in 1997.
BTX is a valuable feedstock for manufacture of petrochemicals and polymers, and is also used as fuel for internal combustion engines. However, its styrene content tends to polymerize and form higher molecular weight compounds that can interfere with processing of BTX as chemical feedstock, or can cause formation of gummy residue that interferes with feeding it for combustion. Hence the presence of styrene in BTX is undesirable when BTX is to be used as petrochemical feedstock or as a liquid fuel for internal combustion engines. The styrene content in BTX is reduced by conversion to ethylbenzene by hydrogenation. Timken et al. in '972 describe a “hydrofinishing” stage in conversion of FCC naphtha to both BTX and high octane gasoline.
However, ethylbenzene and styrene have low value when they are combusted as fuel. BTX from which styrene has been removed has higher value than BTX containing styrene. Further, styrene itself has much higher value when it is recovered for use in manufacture of polymers or petrochemicals when compared to its conversion to ethylbenzene for use as fuel. So far, none of the methods disclosed in, for example, U.S. Pat. Nos. 3,953,300, 4,031,153, and 5,849,982, are effective for removing and recovering styrene from BTX fractions such as pyrolysis gasoline and FCC gasoline, which contain a significant amount of sulphur compounds. This is because styrene is more reactive with hydrogen than thiophenic sulfur in hydrotreating which is the only way to desulfurize the purified styrene stream commercially.
A prior publication Effective Au(III) catalyzed addition of alcohols to alkenes, Royal Society of Chemistry 2007, Chem. Commun. 2007, 3080-3082, describes the preferential formation of C9 ethers in a process of addition of primary alcohols, such as methanol and ethanol, to C8 or C9 aromatic hydrocarbons. See Table 1 (entry 9 versus entry 1).
What is required is a process that removes styrene and methylstyrene from C5-C9 hydrocarbon blends more efficiently than present refining and separation processes. This is done by removing both styrene and methylstyrene prior to the second stage hydrogenation reactor to minimize the formation of EB (from hydrogenation of styrene). The removal increases the capacity of the hydrogenation reactor and makes the down stream separation of xylenes much easier. One such process is described in our co-pending U.S. application Ser. No. 13/373,094, filed Nov. 4, 2011, the disclosure of which is incorporated herein by reference. In this process, styrene is removed by etherification to the corresponding ether, by reaction with a C1-C3 lower alcohol, such as methanol or ethanol in the presence of a suitable acidic catalyst which is selective for etherification. It is noted that the process described therein is restricted to the production of substantially styrene-free C6-C8 aromatic hydrocarbons (BTX).
Accordingly, Applicants have now developed a more versatile process for selectively producing various substantially styrene-, methylstyrene- and ethylbenzene-free C6-C9 aromatic hydrocarbon blends.